Polyphenylene ethers (hereafter abbreviated to PPE), also known as polyphenylene oxides, are heat resistant thermoplastic engineered resins having a high glass transition temperature. In order to improve the moldability, flexibility and elongation of PPE's, they are often used as compositions combined with styrenic resins such as high-impact polystyrenes (modified PPE's), or as compositions combined with elastomers.
Examples of elastomers that can be blended with PPE's include styrenic elastomers such as styrene-butadiene random copolymers, styrene-butadiene block copolymers, styrene-isoprene block copolymers, hydrogenated styrene-butadiene block copolymers (such as SEBS), hydrogenated styrene-isoprene block copolymers (such as SEPS), and α-olefin elastomers such as ethylene-α-olefin copolymers. The blending of styrenic elastomers with PPE's is a widely known technique for softening and improving the impact resistance of PPE's, which is commonly known in the relevant field (Patent Documents 1-4). Among these techniques, those involving non-hydrogenated copolymers of butadiene or isoprene with styrene have poor weather resistance and stability with respect to heat and light, due to the presence of double bonds in the main chain. Additionally, ethylene-α-olefin copolymers have poor compatibility with PPE's. For this reason, hydrogenated block copolymers including hydrogenated styrene-butadiene block copolymers (such as SEBS) and hydrogenated styrene-isoprene block copolymers (such as SEPS) are often employed as compatibilizers. However, the processing required of hydrogenated copolymers (such as SEBS and SEPS) can make them expensive, so there has been a demand for materials not requiring hydrogenation.
In view thereof, resin compositions blending ethylene-styrene (aromatic vinyl compounds) copolymers with polyphenylene ether resins have been considered (Patent Documents 5-9).
However, ethylene-styrene copolymers are statistical copolymers (so-called random copolymers) whose copolymerization is expressed by Bernoulli, first-order and second-order Markov statistics. Therefore, soft copolymers having a low styrene content may have inadequate compatibility with polyphenylene ethers, as a result of which sufficient heat resistance may not be achieved. On the other hand, if the styrene content is high, the glass transition temperature may be close to room temperature, in which case the softness may be inadequate. Thus, resin compositions that are satisfactory in both heat resistance and softness have not been able to be obtained by blending ethylene-styrene (aromatic vinyl compounds) copolymers with polyphenylene ether resins.
On the other hand, as thermoplastic resins aside from polyphenylene ether resins, a method of copolymerizing small quantities of divinylbenzene to ethylene-styrene copolymers and introducing heterologous polymer chains (cross chains) via vinyl groups in the divinylbenzene units, in other words, methods of producing so-called cross-copolymers, and the cross-copolymers obtained by such methods, have been proposed (Patent Documents 10 and 11). Cross-copolymers having polystyrenes as cross chains obtained by the present method can be made to have heat resistance up to near the glass transition temperature of polystyrene (about 100° C.), while retaining the excellent properties of styrene-ethylene copolymers as thermoplastic elastomers. However, there has been a desire for resin compositions capable of further improving their heat resistance while retaining the excellent thermoplastic elastomer properties of cross-copolymers.    Patent Document 1: JP S53-71158 A    Patent Document 2: JP S54-88960 A    Patent Document 3: JP S59-100159 A    Patent Document 4: EP 0 209 874 B1    Patent Document 5: JP H11-181272 A    Patent Document 6: JP 2002-533478 T    Patent Document 7: JP 2000-178388 A    Patent Document 8: JP 2000-198918 A    Patent Document 9: JP H8-3001 B    Patent Document 10: WO 00/37517    Patent Document 11: WO 2007/139116